Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a duplexer (diplexer, triplexer, quadplexer, quintplexer, etc.) for example, connected between an antenna and a transceiver for filtering received and transmitted signals.
Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW) resonators, including film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs), or surface acoustic wave (SAW) resonators. The resonators convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. A BAW resonator, for example, is an acoustic device comprising a stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers). One type of BAW resonator includes a piezoelectric film as the piezoelectric material, which may be referred to as an FBAR as noted above. FBARs resonate at GHz frequencies, and are thus relatively compact, having thicknesses on the order of microns and length and width dimensions of hundreds of microns.
Resonators may be used as band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. With increasing power requirements placed on devices (e.g., mobile phones), ever increasing power demands are placed on filters, and particularly the resonators of the filters. While increasing the active area of a resonator decreases power density, providing an increase in its power handling capability, there is a point of diminishing return. In particular, as the size of the resonator increases, a point is reached where the ability of the resonator to dissipate power is diminished mainly due to a non-uniform strain/stress profile and relatively increased overall thermal resistance compared to smaller active-area resonators. In addition to operating at relatively higher temperatures with increased power, resonators with significantly larger areal dimensions also develop more non-uniform thermal gradients, which weakens the resonator at certain locations in the active area. Ultimately, the power handling capability of the comparatively large active area resonators is limited, and its electrical performance is compromised.
What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above.